1. Field of the Invention
The invention relates generally to aircraft alarm or indicating systems and more particularly to a system with a variable alarm threshold for detecting and providing a warning of impending dangerous windshear conditions.
Windshears may be defined as the rate of change of wind due to the rapid changes in wind velocity and direction that an aircraft experiences with changes in altitude, position, or time, the most dangerous condition occurring during take-off and landing approaches. Windshear may result in changes in the air speed of the aircraft and undesired vertical accelerations. A windshear warning system provides an alarm or other indication to alert the pilot that a dangerous level of windshear exists so that he can take timely corrective action.
2. Description of the Prior Art
U.S. Pat. No. 4,857,922 assigned to the assignee of the present invention, describes an apparatus responsive to both the magnitude of the windshear and the allowable time the windshear can be tolerated at a given magnitude. The sensed magnitude is used to compute the allowable time before providing a windshear alarm. For larger windshear rates, the allowable time is short; conversely, for small windshear rates which exceed a predetermined threshold level, a longer time period is computed before an alarm occurs. This approach minimizes or eliminates nuisance or false warnings of windshear to the flight crew where the aircraft possesses sufficient energy so that the windshear poses no serious threat to safety of flight.
Most current windshear detection systems rely on establishing a given threshold beyond which a measured windshear will result in a warning to the flight crew. These thresholds are, in general, somewhat independent of the actual energy level of the aircraft and therefore may result in an alert of a windshear condition, when, in fact, the aircraft has a surplus if kinetic energy and can easily cope with the measured level of windshear. Such an alert is generally referred to as a "nuisance" alert; that is, an alert which unnecessarily is indicated to the flight crew, since the aircraft has sufficient energy to deal with the windshear magnitude. When the flight crew has been alerted to expect either turbulence or windshear, as by the control tower or an onboard windshear detection system, it is relatively common for the crew to fly the aircraft at a speed greater than the normal landing approach speed. Hence, the ideal windshear detection system should account for any change in kinetic energy to adjust its detection thresholds.
U.S. Pat. No. 4,855,738 provides a system in which the windshear warning threshold is varied as a function of the excess of the approach air speed over the normal approach air speed for the aircraft. This patent suggests, for example, manual input by the pilot of the numerical value of the normal approach reference air speed. However, this is an undesirable feature, as the flight crew may forget to enter the data, thus precluding proper operation of the system, or the flight crew may enter the data incorrectly, thus negating the advantage of the scheme. It is also proposed to compute the normal approach reference air speed in a manner similar to that used for computing the approach air speed as set fourth in flight manuals. That method used parameters including the actual air speed of the aircraft, the acceleration normal to the flight path, flap position, and angle of attack. However, the normal acceleration and angle of attach sensors are affected by turbulence and are sensitive to speed variations. Consequently, fluctuations in computing the normal approach speed result in errors in the windshear warning threshold adjustment.
The present invention relies on a computation of normal approach speed based on the flap position and weight of the aircraft, the latter of which is not generally known save from direct pilot entry into the windshear detection system. The present invention overcomes the shortcomings of the prior art by using a measurement of angle of attack and deducing the aircraft's approximate weight and normal landing approach speed. The difference between the computed landing approach speed and the actual approach speed is used to calculate an "add-on" differential to the windshear system detection threshold. Further, the estimated weight parameter thus derived can be utilized by other aircraft systems, thus obviating the need for a pilot input in those systems.